Nickelation of PCP- and POCOP-Type Pincer Ligands: Kinetics and Mechanism

This report describes the results of a combined experimental and computational investigation on the kinetics and mechanism of the C–H metalation step involved in the formation of PCP- and POCOP-type complexes of nickel. The kinetics of the C–H nickelation reaction was probed through competition studies involving two ligands reacting with a substoicheometric quantity of {(i-PrCN)­NiBr2}n. These experiments have confirmed that metalation is more facile for aromatic ligands 1,3-(i-Pr2PE)2C6H4 vs their aliphatic counterparts 1,3-(i-Pr2PECH2)2CH2 (sp2 C–H > sp3 C–H; E = O, CH2), ligands bearing phosphine moieties vs those with phosphinite moieties (PCP > POCOP), ligands bearing P substituents i-Pr2P vs t-Bu2P and Ph2P, and POCsp2OP ligands 1,3-(i-Pr2PO)2C6RnH4–n bearing electron-donating vs electron-withdrawing substituents (p-OMe ≈ m-OMe > p-Me > m-CO2Me > p-CO2Me > m,m-Cl2). Among the latter, there is a 6-fold difference in C–H metalation rate between ligands bearing p-OMe and p-COOMe, whereas the most readily metalating ligand, 1,3-(i-Pr2PCH2)2C6H4, is metalated ca. 270 times more readily relative to the least reactive ligand, 1,3-(i-Pr2POCH2)2CH2. Density functional calculations indicate that PCP- or POCOP-type pincer ligands react with NiBr2 to generate nonmetalated intermediates that form the corresponding pincer complexes via a two-step mechanism involving an ionic dissociation of the bromide to give a tight ion pair intermediate, followed by bromide-assisted deprotonation of the C–H bond. The type of structure adopted by the nonmetalated intermediates (mono- or dinuclear; tetrahedral, cis or trans square planar) and the energy barriers for the metalation transition states depend on the steric properties of the PR2 moiety. The presence of a base that can neutralize the HBr generated in the metalation step is crucial for rendering the metalation process exergonic. One rationale for the more facile metalation of PCP ligands in comparison to their POCOP counterparts is the greater donor character of phosphine moieties, which allows a more effective stabilization of the coordination and metalation transition states wherein the strongly donor halide ligand is displaced by a much weaker C–H bond donor. The aromatic ligands metalate more readily than their aliphatic analogues for multiple reasons, including the higher ground state energy of the nonmetalated intermediates formed with aromatic ligands, the stronger Csp2–Ni bond formed via metalation, and the more stabilized anionic charge on the C atom being metalated.